1. Field
The present description relates to microelectronic photolithography and, in particular, to determining aerial images for photolithography masks that are modified.
2. Background
In the production of microelectronic and micro-machine devices, such as memory, processors, and controllers, photolithography is used. In photolithography, a photoresist layer is applied to a semiconductor wafer or other substrate. The photoresist is exposed through a mask. The mask exposes or shields different portions of the wafer from light, or some other element. The exposed photoresist is then processed with etching, deposition and other processes to produce the features of the various components, such as circuits and structures, in the wafer that make up the finished product.
The masks are designed using computer design programs that derive an aerial view or image of the wafer based on the electronic circuitry that is to be built on the wafer. The mask is designed to produce this aerial image on the wafer in the particular photolithography equipment that is to be used. In other words the mask must be designed so that when a particular wavelength of light at a particular distance is directed to a wafer through a particular set of optics and the mask, the desired pattern will be illuminated with the desired intensity on the wafer.
The computations required to design the diffractive mask may be complex. For a 193 nm light source, each pixel may be about 100 nm across. For a 4 cm square mask, there may be billions of pixels. If, for example, the features on the wafer are illuminated by diffraction, each pixel may be either a transparent spot on the mask (1), an opaque spot on the mask (0), or a transparent spot that reverses the phase of the light passing through (−1). The use of three different values (+1, 0, −1) allows for greater control over the diffractive effects through the mask. However, with billions of pixels, it further increases the complexity.
Much of the time required for a computer to design a mask is spent evaluating the aerial image that the mask will produce on the wafer and generating a set of pixels that will produce the desired aerial view in the intended photolithography chamber. With complex circuits, the computing time required to design a mask can have a significant impact on the development time for a particular new microelectronic device.
Whenever a circuit element is modified and whenever the design of the mask is improved or optimized, the mask must be recomputed. For a typical optimization process in which the pattern produced by a prototype mask is improved to produce a more accurate pattern, the mask must be recomputed several times to complete the optimization. Each mask computation may require several days to recompute the billions of pixels used for complex circuits. If there are several iterations, then several weeks may be added to a development cycle.